A US Border Patrol agent has been shot dead and another wounded after coming under fire in an area of Arizona used by drug smugglers and human traffickers.

Three agents were responding to a ground sensor that had been tripped about five miles from the Mexican border, near the city of Bisbee, when the shooting began, authorities said.

The wounded agent's injuries are not believed to be life-threatening while the third agent was unharmed.

The agent who was killed was named as Nicholas Ivie, originally from Utah. He is the fourth border official to die in Arizona in the last two years.

Cochise County Sheriff's spokeswoman Carol Capas said: "As they were walking up the trail, they reported taking gunfire.

"We have unknown suspect or suspects at this point."

She said the agents who were shot were assigned to the Brian A Terry Border Patrol Station, named after an agent whose 2010 death was linked to a botched US operation to track guns smuggled to Mexico.

Homeland Security Secretary Janet Napolitano said she was deeply saddened by the "inexcusable crime".

She added that it "reminds us of the risks our men and women confront, and the dangers they willingly undertake, while protecting our nation's borders".

US Border Patrol colleagues of the shot agent

Arizona Governor Jan Brewer said: "Flags will be lowered in honour of the slain agent. Elected officials will vow to find those responsible. Arizonans and Americans will grieve, and they should. But this ought not only be a day of tears.

"There should be anger, too. Righteous anger - at the kind of evil that causes sorrow this deep, and at the federal failure and political stalemate that has left our border unsecured and our Border Patrol in harm's way."

Brewer is a vocal opponent of President Barack Obama's administration on immigration, accusing the government of failing to secure Arizona's border.

In 2010, she signed into law a broad crackdown on illegal immigration, which required police to check the status of anyone stopped who they believe could be in the country illegally.

An estimated 360,000 undocumented people live in the state.

Democratic Representative Ron Barber, who represents the southern Arizona district where the shooting occurred, said in a statement: "We need to redouble our efforts to secure the border and ensure the safety of Border Patrol agents."